Relaxin is a polypeptide hormone which is produced during pregnancy in the ovaries and/or reproductive tract of many species. Structural studies we have conducted with highly purified porcine relaxin indicate that it has a molecular weight of approximately 6,700 and consists of two nonidentical polypeptide chains alpha and beta which are linked by disulfide bond(s). The physiological significance of relaxin remains poorly understood. We have developed a sensitive and precise radioimmunoassay for porcine relaxin which we have employed for the determination of plasma relaxin concentrations in pregnant pigs under experimental conditions designed to provide insight into 1. the role of relaxin throughout pregnancy and at parturition and 2. the factors which influence relaxin secretion. The radioimmunoassay developed with porcine relaxin is specific for pig relaxin and therefore rigorous studies requiring the determination of relaxin and develop a sensitive radioimmunoassay for rat relaxin in order to extend the possibilities for physiological studies involving plasma relaxin levels to a common laboratory species. We propose to locate tissues which are responsive to relaxin in the female rat. We will initially administer low doses of biologically active, radiolabeled relaxin to ovariectomized adult female rats and determine the tissues which bind relaxin by autoradiography and direct counting of excised tissue. In vitro studies will then be conducted in order to determine whether the relaxin binding sites demonstrate the saturability, specificity, and affinity required to be defined as relaxin receptors. Efforts will be made to correlate relaxin binding with biological activity by determining the influence of relaxin on cyclic adenosine 3',5' monophosphate levels in tissues with apparent relaxin receptors. We intend to determine whether preconditioning of the rat with estrogens influences the number of relaxin receptors in the uterus and other relaxin responsive tissues.